PRACTICAL EXERCISES 529 



degrees. .-. w(T'-T") = M(T"-T) + M'(T"-T). Everything in this 

 equation except M' is known, and .*. M', the water-equivalent of the 

 calorimeter, can he deduced, and must be added in all exact experi- 

 ments to the mass of water contained in it. 



Secondly, all the excess of heat in the expired over that in the 

 inspired air is not given off to the calorimeter, for the air passes out 

 of it at a slightly higher temperature than that of the atmosphere. At 

 the beginning of the experiment this excess of temperature is zero. 

 If at the end it is 1 C., the mean excess is 0-5 C. Now, when 

 respiration is carried on in a room at a temperature of 10 C., the 

 expired air has its temperature increased by nearly 30 C. About ^ 

 of the heat given off by the respiratory tract in raising the tempera- 

 ture of the air of respiration would accordingly be lost in such an 

 experiment. But since the portion of the heat lost by the lungs which 

 goes to heat the expired air is only \ of the whole heat lost in 

 respiration (p. 497), the error would only amount to T | 17 of the whole, 

 and this is negligible. 



Thirdly, the air leaves the calorimeter saturated with watery vapour 

 at, say, io - 5, while the inspired air is not 

 saturated for 10 C. Now, the quantity of heat 

 rendered latent in the evaporation of water 

 sufficient to saturate a given quantity of air at 

 40 C. (the expired air is saturated for body 

 temperature) is six times that required to satu- 

 rate the same quantity of air at 10. If, then, 

 the inspired air is half saturated, the error 

 under this head is y 1 ^, or 8 per cent. If the 

 inspired air is three-quarters saturated, the error 

 is ^V or about 4 per cent. If the air is fully 

 saturated before inspiration, as is the case when 

 it is drawn in through a water-valve (Fig. 145) 

 by a tube fixed in one nostril, the only error is 

 that due to the slight excess of temperature of FIG. 145. BOTTLE 

 the air leaving the calorimeter over that of the ARRANGED FOR 

 inspired air. The latent heat of the aqueous WATER-VALVE. 

 vapour in saturated air at 10-5 C. is about ^ more than the latent 

 heat of the aqueous vapour in the same mass of saturated air at 10 

 C., or about -nhrf tne latent heat in saturated air at 40. The error 

 in this case would therefore be under i per cent. The tubes must 

 be wide and the bottle large. 



4. Variations in the Quantity of Urea excreted, with Variations 

 in the Amount of Proteids in the Food. The student should put 

 himself, or somebody else if he can, for two days on a diet poor in 

 proteids, then (after an interval of forty-eight hours on his ordinary 

 food) for two days on a diet rich in proteids. A suitable table of diets 

 will be supplied. The urine should be collected on the six days of the 

 period of experiment, on the day before it begins, and on the day after 

 it ends. Small samples of the mixed urine of the twenty-four hours 

 for each of these eight days should be brought to the laboratory, and 

 the quantity of urea determined by the hypobromite method. The 



34 



